Air flow to conventional automotive spark ignition internal combustion engines is controlled by a butterfly throttle valve secured to a valve shaft which is journalled for rotation in a throttle body. A throttle lever secured to the shaft is operated to position the throttle valve so that the desired air flow is admitted through the throttle body to the engine.
In some applications, an axial load on the shaft can cause the throttle valve to scrape on its bore. The wear which can result tends to admit more air flow through the throttle body than was originally calibrated for a particular throttle position.